Large steam driven turbine generators of the type used to produce electrical power on a large scale are provided with a number of hydraulically operated valves to control the flow of steam to the turbine. These valves, which may number eight or more for any particular turbine-generator set, include control valves for proportional control of steam flow and shutoff valves for fully opened or fully closed operation.
In the event of a sharp decrease or loss of electrical load, it is essential that the steam supply to the turbine be shut off immediately to prevent damaging overspeed conditions. The length of time permitted between a loss of generator load and the closing of turbine steam valves is on the order of 0.1 to 0.2 seconds. Known examples of hydraulic actuator systems which have been developed and applied for achieving these requirements include the hydraulic operating and emergency closing mechanism taught in U.S. Pat. No. 3,495,501 to Kure-Jensen.
Although such actuator mechanisms have proved to be highly reliable in turbine-generator control, they do require periodic maintenance, repair, and testing to preserve and insure continued reliability. During these procedures in the past, it has been necessary to shut down the entire turbine-generator to service any single actuator. This has resulted from the fact that individual actuators have been without means for separate isolation from the common hydraulic system and shutting down one valve has required that they all be shut down.
Accordingly, it is the principal object of the present invention to provide apparatus for isolating an individual steam valve actuator from the hydraulic supply system so that components of the isolated actuator system can be replaced or repaired without a complete shutdown of the turbine-generator.
Other objects, advantages, and features of the invention will become apparent from the description which follows.